This invention relates to a mold for injection molding of toothed power transmission belt having embedded cords as the tensile element.
Up to the present time, toothed power transmission rubber belts have usually been produced using a vulcanizer by the following method. First, canvas for protecting the teeth flanks, cords serving as tensile elements, and unvulcanized rubber sheets are wound in multiple layers around a mold having tooth-shaped grooves on the outer circumferential surface thereof. After that, the assembly is placed in a vulcanizer, and high pressure steam is introduced into the vulcanizer to pressurize and heat the assembly so as to effect cross-linking (vulcanization) of the rubber sheets. Then the tubular molded part together with the mold is removed from the vulcanizer, and the molded part is removed from the mold. The part is cut crosswise into shorter width rings having the desired width and finished to form power transmission belts, the final product.
This production method using a vulcanizer has a problem related to productivity. The production of belts by this method involves many steps since it is necessary, first, to produce rubber sheets with a desired thickness by means of a calender or the like, and then to wind the rubber sheets in layers around the canvas and cords that have been wound around the mold. Furthermore, the cross-linking cannot be completed in a short time. The cross-linking takes much time since the thermal conductivity of rubber itself is low whereas the cross-linking reaction is promoted by the heat transfer from the steam.
The above production method using a vulcanizer also has a problem regarding quality. In case of a rubber sheet material of which fluidity is low even when heated by steam, the material may not be sufficiently forced into the tooth grooves on the mold, resulting in a defective forming of the teeth of the belt. In such a case, it may improve the teeth forming to raise the steam pressure. The practical steam pressure, however, has a limit.
Injection molding may produce good molding of teeth since it allows the use of a high injection pressure. In practice, however, it is difficult to apply injection molding techniques to the production of a belt which is a compound body of rubber, cords, etc. One reason is that the flow pressure of the material being injected into the mold will disturb the alignment of the cords that have been wound in advance around the mold, which will result in the production of belts that always meander or deviate to one side in use. Another problem may be a greater tendency to occur faults such as short shot and weld line.
Injection molding is a highly efficient method of production, and is generally used in the production of rubber vibration isolators and the like. With regard to the production of belts, however, there is only one instance, described in the Japanese Provisional Publication No., SHO.47-11692. The method of production described in the publication is for producing a V-shaped rubber belt, wherein an assembly of a stretching element (rubber) and tensile elements (cords) is molded beforehand and mounted in a ring-shaped recess in a mold, and then molten rubber is injected into the residual space in the recess by injection molding to form a V-shaped belt.
According to the method of production described in the above publication, the cords are integrated with a portion of the rubber before being placed in the recess of the mold. Hence the alignment of the cords will not be disturbed by injection molding. The process, however, is complicated and low in productivity, and does not exhibit the merits of injection molding.